How did shellfish and coral survive acidic oceans in earlier eras?

How did shellfish and coral survive acidic oceans in earlier eras?

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Throughout most of Earth's history, the planet has had more CO2 in its atmosphere, correct? Therefore th pH of the oceans was even lower, right? So how did CaCO3-using animals create their shells back then?

They didn't survive when carbon dioxide levels in the atmosphere passed certain thresholds in the history and increased ocean acidity significantly; leaving less carbonate for marine life to use for shells or skeletons. For example, up to 96% of marine life (including organisms with CaCO3 skeletons) went extinct in the Earth's most severe known extinction event, Permian-Triassic extinction. Another event called Paleocene-Eocene Thermal Maximum caused a mass extinction of benthic foraminifera.

Five times in the Phanerozoic [the past 542 million years (My)], more than three-fourths of marine animal species have vanished in mass extinctions. Each of these events is associated with a significant change in Earth's carbon cycle. However, there are a great many disturbances of the carbon cycle whose imprint in the geologic record is almost exclusively environmental. These other events are relatively benign, with extinction rates barely, if at all rising, above background levels.

Two well-studied examples illustrate these distinctions. The end-Permian extinction [~252 million years ago (Ma)], the most severe mass extinction in the Phanerozoic, plays out over a period of 104 to 105 years; the extinction interval immediately follows a perturbation of the carbon cycle of similar duration. The Paleocene-Eocene Thermal Maximum (~55.5 Ma) is a carbon cycle event of roughly similar time scale, with unambiguous signs of global warming and ocean acidification. It is associated with a significant extinction of benthic foraminifera, but there is no evidence of other major extinctions. What makes these two events so different? The unambiguous co-occurrence of the end-Permian extinction with massive volcanism provides one indication, as does the greater magnitude of the end-Permian carbon isotopic excursion compared to that of the Paleocene-Eocene Thermal Maximum. However, no general distinction, applicable throughout the Phanerozoic, exists.

Thresholds of catastrophe in the Earth system
By Daniel H. Rothman
Science Advances | 20 Sep 2017 : E1700906

(emphasis mine, removed citation numbers)

Here is an excerpt from a relevant study of Knoll et al. about the affects of end-Permian mass extinction on biomineralization which includes the extinction percentages of animals that form skeletons of calcium carbonate and vulnerable to elevated CO2 levels:

Within the marine realm, animals with active circulation and elaborated gills (or lungs) for gas exchange compensate for elevated PCO2 better than organisms that lack these features. Also, animals that normally experience high internal PCO2 , including infauna and metazoans capable of exercise metabolism at high rates, tolerate experimental increases in carbon dioxide differentially well. And among animals that form skeletons of calcium carbonate, species that can exert physiological control over the pH balance of the fluids from which minerals are precipitated fare better than those with limited buffering capacity. Based on these features, Knoll et al. (1996) divided the Late Permian marine fauna into two groups, with high and low expected survival, respectively.

As predicted, 81% of genera in the “vulnerable” group disappeared at the boundary, while only 38% of nominated “tolerant” genera became extinct (Figure 6A). The selectivity is even more pronounced when skeletal physiology is considered alone (Figure 6B; Bambach and Knoll, in preparation). Fully 88% of genera disappeared in groups that formed massive carbonate skeletons but had limited capacity to buffer fluids (sponges, cnidarians, bryozoans, calciate brachiopods). In contrast, groups that built skeletons of materials other than calcium carbonate lost only about 10% of their genera. Intermediate taxa-groups like mollusks that have a substantial commitment to carbonate skeletons but some physiological capacity to regulate the chemistry of internal fluids-lost 53% of their genera. Of these, those genera considered vulnerable on the basis of the other criteria noted above went extinct at twice the rate of those otherwise considered tolerant.

Biomineralization and Evolutionary History
By Andrew H. Knoll
Department of Organismic and Evolutionary Biology
Harvard University
Cambridge, Massachusetts, 02138 U.S.A.

(emphasis mine)


Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environments, some are found in freshwater. In addition, a few species of land crabs are eaten, for example Cardisoma guanhumi in the Caribbean. Shellfish are among the most common food allergens. [1]

Despite the name, shellfish are not fish. Most shellfish are low on the food chain and eat a diet composed primarily of phytoplankton and zooplankton. [2] Many varieties of shellfish, and crustaceans in particular, are actually closely related to insects and arachnids crustaceans make up one of the main subphyla of the phylum Arthropoda. Molluscs include cephalopods (squids, octopuses, cuttlefish) and bivalves (clams, oysters), as well as gastropods (aquatic species such as whelks and winkles land species such as snails and slugs).

Molluscs used as a food source by humans include many species of clams, mussels, oysters, winkles, and scallops. Some crustaceans that are commonly eaten are shrimp, lobsters, crayfish, and crabs. [3] Echinoderms are not as frequently harvested for food as molluscs and crustaceans however, sea urchin roe is quite popular in many parts of the world, where the live delicacy is harder to transport. [4] [5]

Drilling Into Ocean History

Like forensic detective work, coral coring has become a reliable way to add detail and credibility to theories about past events&mdashor to prove that they happened at all. It&rsquos easy to forget that no one was even sure corals set down annual growth rings until the 1970s. That was when a team of University of Hawaii geophysicists visited Enewetak Atoll in the South Pacific.

Enewetak was an unassuming island with an unusual history: The United States tested nuclear bombs there on various dates in the 1940s and &rsquo50s. The Hawaii researchers were curious to see whether coral skeletons near Enewetak would show evidence of this radioactivity. If the coral core layers contained radioactive elements with a known half-life, it would be possible to calculate almost exactly when each growth ring was made. &ldquoThey took a slice of a massive colony, put it on [light-]sensitive paper in a darkroom for a month, and they saw a series of radioactive bands,&rdquo Lough said. The spacing of the bands on the paper hinted that there might be more to discover within the hidden structure of the coral, suggesting a further test was in order. &ldquoThey got in touch with the local doctor and said, &lsquoWould you mind X-raying our coral slice?&rsquo&rdquo

When the coral slices were put in the X-ray scanner, a distinctive series of light and dark growth rings became visible, reflecting the density of the calcium carbonate that made up the coral skeleton. Dating the radioactive elements in the skeleton revealed that a double set of rings was laid down each year: a larger, more porous ring and a narrower, denser ring. In a 1972 Sciencepaper, the researchers dubbed the cores &ldquocoral chronometers,&rdquo alluding to their utility as natural timepieces. Other scientists have since reported that corals lay down larger growth rings during wet seasons when temperatures are more moderate, and smaller rings during dry seasons when conditions are more extreme.

Coral species grow between 0.3 and 10 centimeters per year, but a general rule of thumb is that a 100-centimeter-long core sample, for instance, supplies a record of about 100 years of that coral&rsquos history. Often it is the most recent 100 years, but not always. Fossilized corals may contain sequences of growth rings that date back as far as the last interglacial period, more than 100,000 years ago. X-ray scans are still used today to assess the relative density of coral growth rings, which reflects the climatic conditions at the time the rings were created. But marine scientists have worked steadily to discover the significance of other coral core properties as well.

One of the richest stores of data inside a core, coral detectives are finding, is its year-by-year record of trace elements in ocean water. Coral polyps take in ocean water to extract minerals they need to build their skeletons, so each carbonate layer contains tiny amounts of whatever was in the water when the layer was created. While coral growth rings are &ldquonot as nice and tight as tree rings, due to the complex internal shape of the skeleton,&rdquo said Gregory Webb, a University of Queensland paleontologist, &ldquothey do record the chemistry of the water they grow in.&rdquo

Tests of coral core composition, therefore, allow scientists to chart levels of many different compounds in an ocean zone from one year to the next. This can yield insights into planetary processes that seem to have little to do with coral. Marine scientists at China&rsquos Guangxi Key Laboratory recently deduced the strength of East Asian winter monsoons over the past 150 years by measuring levels of rare earth elements, such as lanthanum and cerium, in each layer of a Poritescoral core. These rare earth elements come from swirls of dust deposited during the winter storms, so the elements&rsquo prevalence is a reliable gauge of storm intensity.

Likewise, coral core tests are uncovering historical evidence of human-caused pollution that is far more detailed than any found before. Lough and her colleagues are sampling modern cores from the Great Barrier Reef and testing growth layers for levels of toxic metals such as lead and cadmium, which often come from industrial production. Developers might build a port, dump sediment onto a coral reef, and insist their intervention has no effect on the ocean&mdashbut, as Lough pointed out, &ldquothe coral cores are unbiased observers of how the environment is changing.&rdquo

Coral cores also supply some of the only reliable records of ocean temperatures during the years before official measurements were taken. When waters are cooler, corals use more of the element strontium to supplement the calcium carbonate they use to build their skeletons. By calculating the ratio of calcium to strontium in each layer of a coral core, researchers can determine what the ocean temperature was when that layer was created.

Using this technique on coral cores from the tropical Pacific waters around Ecuador&rsquos Galápagos Islands, the University of Arizona geoscientist Gloria Jimenez and colleagues recently assembled a detailed record of water temperature changes from 1940 to 2010. Previous temperature records for the area were spotty, and they seemed to indicate that ocean warming in the Galápagos was limited due to incoming cool currents from the depths. But Jimenez&rsquos coral core data told a different story: Waters in the area had actually been warming since the late 1970s, with a spike in the early 1980s when warm El Niño currents traveled through. This steady warming trend means that reefs around the Galápagos may be in more peril than previously thought.

Beneath the modern coral formations Jimenez studies is another trove of data bound up in fossilized coral cores. Depending on their state of preservation, these cores allow researchers like Webb to extend ocean temperature records more than 100,000 years into the past. Webb has a customized boat, the research vessel D Hill, that features a drilling platform for taking core samples from ancient strata below the Great Barrier Reef.

After Webb and his team recover fossilized coral cores, they can determine a core&rsquos age by using uranium-thorium dating. Mass spectrometer analysis shows how much of the trace uranium in the core&rsquos layers has decayed to thorium, and the ratio between the two elements is used to calculate each layer&rsquos approximate age. Like Jimenez, Webb uses strontium-calcium ratios to calculate ocean temperatures at the time each coral band was made, and he uses his fossil cores to track the prevalence of trace elements in prehistoric waters. &ldquoWe have been able to recover cores from the entire Holocene,&rdquo Webb said, referring to the current geological epoch, which started about 12,000 years ago. &ldquoWe can begin to compare climate and water-quality issues in the same reef, at the same exact spot, but 100,000 years apart.&rdquo

Webb&rsquos fossil core analysis is also turning up new evidence of ancient geologic processes. During a recent trip to Heron Reef, a region of the Great Barrier Reef off the coast of Australia, he and his team encountered a glitch. The team&rsquos rig is capable of drilling 30 meters into the seafloor, and one day, they calculated that they would soon hit layers dating back to the last interglacial period of the Pleistocene epoch, more than 100,000 years ago. But they never made it all the way. &ldquoWe thought we were going to hit the Pleistocene at about 15 meters,&rdquo Webb recalled. &ldquoWe had bets on what that depth would be&mdashsomeone took 12, someone took 14. Next thing you know, we&rsquore at 22 and we hadn&rsquot hit it yet. We just happened to drill into a valley, and we didn&rsquot expect that at all.&rdquo

As it turned out, the coral core contained a layer dating back to the last ice age, when sea levels were as much as 130 meters lower and the entire Great Barrier Reef structure was above the waves. Wind, rain and running water carved the exposed limestone at that site into a deep depression surrounded by high, steep, craggy hills. When sea levels rose again, currents and waves filled the submerged valley with sediment particles, and that terrain became the foundation for new coral reefs growing at the site. This discovery helped the researchers conclude that the shape of modern reefs isn&rsquot typically determined by the shape of previous reefs or geologic structures on which they are growing, as some scientists thought. Sediment accumulation can cover up the contours of the older structures and provide a flatter surface for the new reefs to grow on. Meanwhile, the highest points on coral reefs can only grow as high as sea levels let them, which means they&rsquore flattened from the top as well.

The shifting movements of the sea have always played an integral role in shaping these unique ecosystems, as a study in Nature Geosciencereleased just this week further attests. Jody Webster of the University of Sydney, Bryan Lougheed of the Institute Pierre Simon Laplace in France, and their colleagues extracted a variety of ancient coral cores from beneath the Great Barrier Reef. Analysis of the skeletal matter and sediments in the cores showed that sea-level changes had killed parts of the reef five times in the past 30,000 years&mdashsometimes when the reefs were exposed to the air, and sometimes when sediments in rising waters blocked light from reaching the reef. The reef regrew in each case, however, because of coral polyps that migrated in from elsewhere, and its live coral formations moved around over time to take advantage of the best water and light conditions available.

The unique structural makeup of each coral layer in a core sample also supplies clues about other stresses the coral encountered as it was forming, whether that happened dozens of years ago or thousands. When oceans are relatively acidic due to dissolved carbon dioxide from the atmosphere, for instance, corals completely change their growth habits, as Woods Hole Oceanographic Institution researchers reported this year in the Proceedings of the National Academy of Sciences.

The Woods Hole team of marine scientists, including the graduate student Nathaniel Mollica and the geologist Anne Cohen, analyzed samples of modern Porites coral cores from waters near Panama, Palau, Taiwan and the Dongsha Atoll in the South China Sea. They put each coral core in a computed tomography (CT) scanner, a specialized X-ray device that reveals growth patterns and density differences deep inside the coral.

By comparing these coral core records to water samples obtained from each site, the scientists demonstrated that higher acid levels during past eras gave rise to distinct structural anomalies. Corals in more acidic waters grew at about the same speed as other corals, but the structure of the acid-exposed corals was different, with gaps like the bubbles in pancake batter. The reason for this is that when carbon dioxide dissolves into ocean water, it attaches to free carbonate ions in the water. As a result, fewer carbonate ions are available for coral polyps to extract from the water, so the polyps can&rsquot produce as much calcium carbonate.

Over time, this deficiency leads to thinner, more porous coral skeletons. &ldquoBasically, we see all these empty spaces [and] hollow areas inside,&rdquo said Weifu Guo, a geochemist on the research team. Such delicate skeletons are more apt to crumble under a storm surge or the crashing of waves&mdashand in turn, that crumbling can imperil other life on the reef, including algae that grow food for corals and fish that depend on corals for sustenance.

FAQs about Ocean Acidification

Ocean acidification is a new field of research in which most studies have been published in the past 10 years. Hence, there are some certainties, but many questions remain. Ocean acidification is also a multi-disciplinary research area that encompasses topics such as chemistry, paleontology, biology, ecology, biogeochemistry, modeling, and social sciences. Furthermore, some aspects of ocean acidification research, for example the carbonate chemistry, are intricate and counterintuitive. For these reasons, the media and the general public find some scientific issues or results confusing.

The U.S. Ocean Carbon and Biogeochemistry (OCB) program, supported by the European Project on Ocean Acidification (EPOCA) and the UK Ocean Acidification Research Programme, has compiled a list of frequently asked questions (FAQs). These questions were widely distributed to the research community with the request to draft concise replies summarizing current knowledge, yet avoiding jargon. The replies were then subject to an open peer-review and revision process to ensure readability without any loss of scientific accuracy. The response of the community was enthusiastic. In total, 27 scientists from 19 institutions and 5 countries contributed to the whole process.

We do hope that this FAQ list will prove useful and would like to point out that it is an on-going process. Anyone is invited to seek clarification or send comments to Sarah Cooley. The list will be revised periodically using this input.

Joan Kleypas and Richard Feely (OCB), Jean-Pierre Gattuso (EPOCA), and Carol Turley (UK Ocean Acidification Research Programme)

The name "ocean acidification"

The ocean is not acidic, and model projections say the oceans won?t ever become acidic. So why call it ocean acidification?
Ocean acidification refers to the process of lowering the oceans’ pH (that is, increasing the concentration of hydrogen ions) by dissolving additional carbon dioxide in seawater from the atmosphere. The word “acidification” refers to lowering pH from any starting point to any end point on the pH scale. This term is used in many other scientific areas (including medicine and food science) to refer to the addition of an acid to a solution, regardless of the solution's pH value. For example, even though seawater's pH is greater than 7.0 (and therefore considered “basic” in terms of the pH scale), increasing atmospheric CO2 levels are still raising the ocean's acidity and lowering its pH. In comparison, this language is similar to the words we use when we talk about temperature. If the air temperature moves from -40°C to -29°C (-40°F to -20°F), it is still cold, but we call it “warming.” — James Orr, Senior Scientist, Laboratory for the Sciences of Climate and Environment, France Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA Robert Key, Research Oceanographer, Princeton University, USA

Would dissolving all the CO2 released by burning all the world?s fossil fuel reserves ever make the seas acidic?
No. The fundamental chemistry of the ocean carbon system, including the presence of calcium carbonate minerals on the ocean floor that can slowly dissolve and help neutralize some of the CO2, prevents the oceans from becoming acidic on a global scale. — Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA

Is ocean acidification just another name for climate change?
No. While ocean acidification and climate change share a common cause (increases in CO2 in the atmosphere), climate change encompasses the effects associated with changes in the Earth’s heat budget (due to the greenhouse effect of CO2 and to a lesser extent other climate reactive gases), which cause global warming and changes in weather patterns. Ocean acidification specifically refers to the lowering of ocean pH resulting from its absorption of human-released CO2 from the atmosphere. Ocean acidification does not include the warming of the ocean. — Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA

Ocean carbon chemistry and pH

The equations showing CO2 reacting with water look like they generate more, not less carbonate. How does ocean acidification decrease the amount of carbonate ions in seawater?
This is a common point of confusion, because step-by-step equilibrium equations describing the carbonate system in seawater do not capture the dynamic chemical environment of seawater. There are several reactions that can occur between carbon dioxide (CO2), water (H2O), carbonic acid (H2CO3), bicarbonate ion (HCO3 - ), and carbonate ion (CO3 2- ). One of the possible reactions does create carbonate ions and lowers pH:

However, at the current ocean pH level, another reaction also occurs that consumes carbonate ions and does not change pH:

The second equation describes the reaction that occurs most often in the modern oceans, but the first reaction also occurs, so the resulting overall change is a decrease in carbonate and a decrease in pH. — Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA

Will CO2 really decrease ocean pH all that much?
Scientists estimate that surface ocean pH has fallen by about 0.1 pH unit from preindustrial times to today. Because pH is a measure of hydrogen ion concentration and the the pH scale is logarithmic — for every drop of 1 pH unit, hydrogen ion levels increase by a factor of 10 — a 0.1-unit pH drop is equivalent to about a 26% increase in the ocean hydrogen ion concentration. If we continue on the expected trajectory for fossil-fuel use and rising atmospheric CO2, pH is likely to drop by 0.3-0.4 units by the end of the 21st century and increase ocean hydrogen ion concentration (or acidity) by 100-150% above what it was in preindustrial times. — Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA

It seems impossible to acidify the oceans, given how salty they are. How could CO2 overcome all that salt?
When acids and bases neutralize each other in a laboratory experiment, salt and water form. But in the ocean, the major ions that make seawater “salty” (like sodium, chloride, and magnesium) have come from rock weathering, which provides a balanced amount of positive and negative ions to the seas over many millennia. Variations in ocean pH on shorter time scales of decades to centuries are controlled by weak acids and bases, like bicarbonate or borate. Of these weak acids and bases, the dissolved forms of CO2, known as carbonic acid, bicarbonate, and carbonate, have the largest impact on global ocean pH variations because their concentrations are changing quickly relative to other ions in the ocean. — Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA

If the ice caps melt and freshwater is added to the ocean, won?t this simply dilute the acidity?
Fresh water from melting ice caps dilutes the concentrations of all the various components of the carbonate system in seawater (described above), as well as the total alkalinity and salinity (both of which affect pH). For example, a liter of “typical” Arctic seawater (temperature, 5°C salinity, 35 total alkalinity, 2244 micromoles/kilogram) that is exposed to today’s atmospheric CO2 level of 390 ppm has a total carbon content of 2100 micromoles/kilogram and a pH of 8.04 (total scale, here and below). Adding a kilogram of freshwater to the kilogram of seawater would dilute the salinity, alkalinity, and carbon content to half of what they were, and the initial pH would increase to 8.21. However, that seawater is out of equilibrium with the atmosphere (it now has a pCO2 of 151 ppm, while the pCO2 level of the overlying atmosphere is 390 ppm) and so it will absorb CO2 until the seawater pCO2 also equals 390 ppm, at which point the pH will have dropped to 7.83.— Richard A. Feely, Senior Scientist, NOAA Pacific Marine Environmental Laboratory, USA Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA

Won?t the CO2 outgas as the oceans begin to warm up, therefore cancelling out the problem?
The CO2 content of the surface waters of the oceans responds to both changes in CO2 content of the atmosphere and changes in temperature. For example, if ocean temperatures were not changing, a doubling of preindustrial CO2 levels (from 280 to 560 ppm) would cause an increase in the total amount of dissolved carbon in the surface ocean from about 2002 to 2131 micromoles/kg of seawater (assuming salinity = 35, temperature =15°C, and alkalinity = 2300 micromoles/kg). If ocean temperatures warmed by 2°C over that period, then less carbon would be taken up (the increase would be from 2002 to 2117 micromoles/kg). Thus, a 2°C increase in temperature results in about a 10% decrease in carbon uptake in surface waters. The expected warming of the oceans also may alter ocean circulation, further reducing their capacity to absorb CO2 from the atmosphere, but the excess CO2 will still remain in the atmosphere and drive further acidification. For pH, the net effects of climate warming on atmospheric CO2, CO2 solubility, and chemical speciation approximately cancel out. — Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA

Measurements and observations

How do we know what ocean pH was in the past even though the pH scale was not introduced until 1909?
When ice sheets build up into glaciers, air bubbles become trapped in the freezing ice. Scientists have analyzed the CO2 concentration of air in these bubbles and have developed a record of the atmospheric CO2 concentration in the recent past. Because large parts of the surface ocean CO2 concentration remains roughly in equilibrium with the atmospheric CO2 concentration, the ocean CO2 content can be calculated from these air bubbles, and ocean pH can also be calculated. In fact, the ice core record shows that the atmospheric CO2 concentration has never been higher than about 280 ppm during the last 800,000 years, creating conditions leading to an average preindustrial surface ocean pH of ca. 8.2. — Jelle Bijma, Biogeochemist, Alfred Wegener Institute for Polar and Marine Research, Germany

What evidence suggests that ocean acidification is happening and that it results from human activity?
Scientists have collected semi-continuous records of seawater pCO2 and pH over the last 20-30 years in the Pacific and Atlantic Oceans. These time-series records from near Hawaii, Bermuda, and the Canary Islands show that seawater pCO2 is mirroring the increase in atmospheric CO2 and that ocean pH is decreasing. Other measurements of the CO2 content in the North Pacific Ocean, conducted in 1991 and again in 2006, show that the CO2 content in the North Pacific Ocean has increased in accordance with rising atmospheric CO2 concentrations. — Carol Turley, Senior Scientist, Plymouth Marine Laboratory, UK Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA

How do we know what ocean pH was tens of millions of years ago?
To estimate physical or chemical parameters such as temperature or pH for periods before instruments were available, scientists use so-called proxy parameters or “proxies,” which are measurable parameters that can be related to desired but unobservable parameters. For instance, marine calcifying organisms incorporate many other elements into their hard shells and skeletons besides the calcium, carbon, and oxygen in calcium carbonate. When the hard parts of these organisms that are preserved in sediment are analyzed, the additional elements provide information about environmental conditions during the animal's lifetime. Historical ocean pH values and changes can be studied using the concentration of the element boron and the ratio of its stable isotopes (δ 10 B and δ 11 B) in marine carbonates. Additional geochemical evidence and modeling provide strong evidence that the average surface ocean pH has not been much lower than about 8.2 for millions of years. — Jelle Bijma, Biogeochemist, Alfred Wegener Institute for Polar and Marine Research, Germany

How do OA's effects relate to those of other human activities?
Other human activities certainly are affecting seawater chemistry and the ocean’s acid-base balance, but not nearly to the extent of atmospheric CO2-driven acidification. Acid rain, which contains sulfuric and nitric acids originally derived from fossil fuel combustion, falls on the coastal oceans. The impact of acid rain on surface ocean chemistry may be important locally and regionally, but it is small globally and its total effects equal only a few percent of the changes driven by rising atmospheric CO2. Coastal waters are also affected by excess nutrient inputs, mostly nitrogen, from agriculture, fertilizers and sewage. The resulting chemical changes lead to large plankton blooms, and when these blooms collapse and sink below the surface layer the resulting respiration from bacteria leads to a drawdown in seawater oxygen and an increase in CO2, which decreases pH even more in subsurface coastal waters.

One of the major differences between OA and these types of human effects is that OA’s influence is truly global in scale, affecting pH-sensitive and calcifying organisms in every ocean basin from the equator to the poles. At present, the effects are restricted primarily to the upper 200-500m of the ocean, but every year the effects penetrate to deeper depths. Many of the other impacts of human activities are more local in nature.— Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA Chris Langdon, Associate Professor, University of Miami

Geological buffering

If glacial runoff increases and rock flour is carried to the oceans, will this provide alkalinity to the oceans and offset OA somewhat?
The weathering of continental rocks does increase the alkalinity of seawater and increases its ability to counteract pH decreases, but neutralizing all of the CO2 from human activity that is entering the oceans with this process alone would take hundreds of thousands of years. Therefore, on the time scales of importance to humankind (decades to centuries), these processes are not fast enough to significantly buffer ocean acidification. — Richard A. Feely, Senior Scientist, NOAA Pacific Marine Environmental Laboratory, USA Jelle Bijma, Biogeochemist, Alfred Wegener Institute for Polar and Marine Research, Germany

As the oceans become more acidic, more calcium carbonate minerals underwater will dissolve. Will that offset ocean acidification?
The dissolution of calcium carbonate minerals in the water column and in the sediments does increase the alkalinity of seawater, which offsets the decreased pH and carbonate ion concentrations associated with ocean acidification. However, as with rock weathering, this process is slow and would take thousands to tens of thousands of years to neutralize all of the CO2 from human activity that is entering the oceans. Over the decades to centuries that affect human communities, these processes are not fast enough to counteract CO2 invasion into the ocean, and so the chemical changes associated with ocean acidification will last for several centuries. — Richard A. Feely, Senior Scientist, NOAA Pacific Marine Environmental Laboratory, USA

OA and photosynthesis

Photosynthesis is expected to rise with ocean CO2 levels, and corals contain photosynthesizing algae, so won't corals benefit from rising CO2?
The photosynthesis of some, but not all, algae increases when CO2 rises to levels projected for the end of this century (700-800 ppm). The single-celled algae called zooxanthellae that live within coral animals’ cells are some of the algae whose photosynthesis does not significantly increase at projected future CO2 levels. Normally, zooxanthellae and corals maintain a delicately balanced symbiosis, in which the zooxanthellae transfer photosynthetically formed carbon-based nutrition to the coral host and provide an important source of carbon for the coral and for coral calcification (skeleton building). If the algae within the corals’ cells do too well and their numbers greatly increase, the transfer of nutrition to the coral host can be disrupted. So even if zooxanthellae photosynthesis were to increase under high CO2, this does not necessarily benefit the corals. In the great majority of experiments, coral calcification rate decreases when the CO2 level increases, so it is clear that the rise in CO2 is decreasing the corals’ ability to build their skeletons rather than protecting them by altering zooxanthellae photosynthesis. — Chris Langdon, Associate Professor, University of Miami, USA Anne Cohen, Research Specialist, Woods Hole Oceanographic Institution, USA

If photosynthesis increases with ocean CO2 levels, won?t phytoplankton and seagrasses do better?
Communities of organisms found near shallow near-shore volcanic CO2 vents demonstrate that certain microalgae, seaweeds and seagrasses grow very well in areas that experience long-term exposure to elevated CO2. However, this work also shows that coastal ecosystems are degraded due to the long-term effects of ocean acidification. Biodiversity is lost: groups of organisms such as coralline algae gradually disappear as pH falls, and they are replaced by thriving stands of invasive algae. This raises concerns that ocean acidification will allow alien algae to proliferate and disrupt coastal habitats. Jason Hall-Spencer, Lecturer, University of Plymouth, UK

An increase of CO2 in seawater increases growth of photosynthetic algae? Isn't that a good thing?
The growth and photosynthesis of certain marine phytoplankton and plant species may increase with higher CO2 levels, but this is by no means a general rule. For other species, higher CO2 and rising acidity will have either negative or neutral effects on their physiology. Therefore some marine phytoplankton and plants will be “winners,” while others will be “losers.” This means that instead of benefiting all impartially, future acidification will instead probably cause major shifts in the species composition of ocean phytoplankton communities. Some of the experiments that have been done so far suggest that the likely new dominant phytoplankton species in the future acidified ocean may be less able to support the productive food chains that we presently rely on to support healthy ocean ecosystems and fisheries resources. — David Hutchins, Professor of Marine Environmental Biology, University of Southern California, USA

OA and calcification

Why does adding CO2 to home aquaria benefit animals, but in the ocean, adding CO2 leads to harmful acidification?
Freshwater fish and plants tend to be more tolerant of lower pH and wider pH changes overall because fresh water contains low alkalinity, which means that the water chemistry does not minimize pH changes (i.e., it does not have the “buffering capacity”) the way that seawater chemistry does. The natural variability of pH in lakes and rivers is also higher than in the ocean. Freshwater organisms have evolved special mechanisms that allow them to thrive in these more acidic and variable conditions for example, freshwater plants may benefit from higher CO2.

In saltwater aquaria, corals and fish require a more narrowly balanced pH and owners often add carbonate “hardeners” to increase the water's alkalinity and maintain the pH between 8.0 and 8.4. Devices called “calcium reactors” bubble CO2 gas through crushed calcium carbonate (usually crushed coral), which releases calcium and carbonate ions into the salt water, providing the high-alkalinity, calcium-rich waters that aquarium corals and other calcifying organisms need to continue healthy growth. Unfortunately, these types of devices cannot be used to solve ocean acidification on a global scale, because of the vast amounts of crushed calcium carbonate that are required to carry out the process in the world’s oceans. — Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA Michael Holcomb, Postdoctoral Research Associate, Centre Scientifique de Monaco, Monaco

Shellfish can survive in fresh water where pH can drop as low as 5, so what?s the problem?
Organisms that live in fresh water or in salt water with lower pH have developed adaptive mechanisms that allow them to survive under those conditions. In contrast, marine shellfish that have evolved in seawater with a higher and less variable pH are more susceptible to changes in pH. A good example of this is the natural shift in marine organisms to freshwater organisms living along estuaries. A marine shellfish, Thais gradata, that is found along estuaries tends to have higher rates of dissolution at the freshwater end of the estuary, where pH is lower and varies widely, than it does at the seawater end of the estuary where pH is higher and varies less. — Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK

Why does increasing the dissolved CO2 concentration in seawater affect shell building in marine organisms?
Dissolving CO2 in seawater causes a suite of changes in the carbonate system in seawater: the concentrations of dissolved CO2, total dissolved inorganic carbon, and the bicarbonate ion increase, while pH, carbonate ion concentration, and calcium carbonate saturation state decrease. One or several of these changes may affect shell building in marine organisms. The formation of skeletons or shells in most marine organisms is an internal process where most organisms appear to convert bicarbonate to carbonate to form calcium carbonate. But because this conversion creates protons (hydrogen ions), the organisms must exert energy to expel the hydrogen ions into the external environment (seawater). One hypothesis as to why ocean acidification can cause slower calcification rates (and there are several) is that as seawater pH decreases, the organisms must exert more energy to rid themselves of the protons produced by calcification --- they are simply working against a steeper gradient. This explains why many calcifying organisms have lower calcification rates when they are physiologically taxed by other stresses (e.g. lack of food) that is, the added stress leaves the organisms with less energy for calcification. Ocean acidification can also indirectly affect shell formation through physiological impacts, such as changes in an organism's respiration rate, which can impact energy budgets and thus alter the animal's ability to produce shell material. While some organisms may grow their shells at normal rates under ocean acidification, the exposed parts of the shell may dissolve more quickly, so that the organism may need to spend more energy in shell maintenance, and less in reproduction or other life activities. — Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK Anne Cohen, Research Specialist, Woods Hole Oceanographic Institution, USA Joan Kleypas, , Scientist III, National Center for Atmospheric Research, USA

Scientists have shown that lobster shells (and shells of other food-providing animals) get thicker when living in water with higher CO2, so why should we be worried about OA?
At least one experimental study showed that the shell mass of several crustaceans, including lobsters, reared in culture for 60 days actually increased with increased CO2. Shell-making requires energy, so increased shell mass almost certainly occurred hand-in-hand with reduced energy for other functions like growth and reproduction. Also, lobsters and other crustaceans make shells using both calcium carbonate and chitin in a different mechanism than other marine organisms. They shed their shell periodically, rather than growing constantly throughout life, and they are thought to retain many of the minerals from their old skeleton to put into the new skeleton. Energy and mineral budgets were not monitored in the above study, so how OA affects the overall health and longevity of these organisms is still not known. — Anne Cohen, Research Specialist, Woods Hole Oceanographic Institution, USA Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK

Individuals and Ecosystems

Won't organisms and ecosystems adapt if some calcifying species leave an area?
The rate of human-driven ocean acidification is about 100 times faster in the surface ocean than that experienced by marine ecosystems globally for tens of millions of years. Different ecosystems will respond differently. In some ecosystems, such as coral reefs, the calcifying organisms form the fundamental architecture of the ecosystem so if they disappear, the ecosystem could disappear. In other ecosystems where calcifiers play a less important role, it is less clear what impact the loss of calcifying species might have on the ecosystem. During profound rapid changes in ocean chemistry like present-day ocean acidification, organisms respond in one of 3 ways: acclimation, adaptation, or extinction. If most species acclimate rapidly, the biodiversity and function of marine ecosystems may be relatively unchanged. Evolutionary adaptation, however, is linked to generation time, meaning that long-lived species that mature slowly will have fewer opportunities to produce offspring more resistant to the rapidly changing environmental conditions. Even species that reproduce more quickly may not be able to adapt for example, at the edges of regions with favorable temperatures and water chemistry, corals have been trying to adapt to lower carbonate ion concentrations for many millions of years, but they have not been able to succeed in outcompeting algae and other non-calcifying species there. It seems unlikely, therefore, that corals could succeed in adapting to new temperatures and water chemistry in a few decades to respond to OA. If OA drives large shifts in the abundance of key organisms in food webs, or significant rates of extinction, we can expect important changes in the function of ecosystems--- how energy and material flow from primary producers like plankton to top predators like fish and mammals.

Ecosystems are complex networks of interactions among biological organisms and the environment, and it is difficult to predict the full ecological impacts of changing any of those links. We know from CO2 vent studies that OA affects biological species differently and the mix of marine species shifts, leading to lowered biodiversity and a change in the overall functioning of ecosystems. We depend on a whole range of marine ecosystem services, including food from fisheries, income from tourism and recreation, and oxygen and nutrient recycling from biogeochemical processes all of these services could be altered, and in many cases degraded, by ocean acidification. Imagine, for example, the economic effects of the disappearance of sea urchins from Japanese fisheries or the decline in fish larvae of commercially important species. Furthermore, decreasing or disappearing calcifying organisms will affect (1) the chemical environment, (2) other calcifying and non-calcifying organisms that may depend on them (e.g., many organisms and hundreds of millions of people depend on coral reefs), and (3) the reservoir of carbon on Earth (the “rock” produced by calcifying organisms falls on the ocean floor to form massive “chalky” deposits that lock away some carbon into geological structures). Just like a neglected aquarium that gives way from fish and shellfish to algae, marine ecosystems may adjust, but they might then be populated by species that are less useful or desirable to humans, making the traditional resources and services provided by the changed ecosystems unavailable, different from before, or unpredictable.— Debora Iglesias-Rodriguez, Lecturer, National Oceanography Centre of the University of Southampton, U.K Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA Steve Widdicombe, Benthic Ecologist, Plymouth Marine Laboratory, UK Jim Barry, Senior Scientist. Monterey Bay Aquarium Research Institute, USA Ken Caldeira, Senior Scientist, Carnegie Institution for Science, USA Jason Hall-Spencer, Marine Biology Lecturer, University of Plymouth, UK

Will ocean acidification kill all ocean life?
No. However, many scientists think that ocean acidification will lead to important changes in marine ecosystems. This prediction is largely based on geologic history: millions of years ago, marine ecosystems experienced rapid changes during ocean acidification events, including some species extinctions (see “OA in Geologic History” below). Today, some species and the ecosystems they sustain are threatened by ocean acidification, particularly in combination with other climate changes such as ocean warming. Examples include tropical corals, deep-sea corals, and swimming snails. These species play key roles in the oceans either because they build three-dimensional structures, which host a considerable biodiversity, or because they are key components of the food chain. Some species that build calcium carbonate structures, such as coral reefs, also provide key services to humans by providing food, protecting shorelines, and supporting tourism. Evidence for the ecological effects of ocean acidification today can be found at “champagne sites,” locations where volcanic CO2 vents naturally acidify the water and small CO2 bubbles rise through the water column. At one of these sites around the Island of Ischia (Italy), for example, biodiversity is reduced by 30% at the acidity level that matches the level expected globally in 2100. — Jean-Pierre Gattuso, Senior Scientist, Centre National de la Recherche Scientifique and Université Pierre et Marie Curie-Paris 6, France Jason Hall-Spencer, Marine Biology Lecturer, University of Plymouth, UK

Will warming and acidification balance out responses from organisms?
In principle, there may be some benefit from warming for the calcification process, because precipitation of calcium carbonate is enhanced by temperature up to a certain threshold. However, organisms are accustomed to living in a limited thermal range and are performing less well in temperatures outside of this range. In many marine areas, organisms (calcifiers and non-calcifiers alike) are already exposed to temperatures reaching the upper end of their thermal windows. Pilot studies on crab and fish have demonstrated that exposure to CO2 levels expected if CO2 emissions continue to increase reduces the animals’ capacity to tolerate extreme temperatures. Studies on corals have also shown that CO2 enhances thermal sensitivity. In this case it encourages the likelihood of bleaching events triggered by warming. Overall, it appears that ocean acidification may enhance the sensitivity of organisms to climate warming. — Hans-Otto Pörtner, Professor, Alfred Wegener Institute, Germany

Will adult organisms be safe if they are able to survive the effects of ocean acidification when they are very young and susceptible?
For common marine organisms, the gametes, eggs, various larval stages, juveniles, and adults may be affected differently by ocean acidification because they have different tolerances and coping strategies to environmental stress. In some cases, the early life stages may be more susceptible to stress, while in other cases, the adults may be. Experiments are necessary on all life stages to understand the full effects on an organism and to highlight stages that represent weak links. It is also important to consider ocean acidification's lifelong impacts on survival and reproduction. In general, early life history phases (gametes, larvae, juveniles) are expected to be more sensitive to ocean acidification than adults. If fewer young organisms survive to adulthood, population size will clearly be reduced. Ongoing stress usually limits the success of organisms – for example, stressed organisms grow slower and smaller, stressed predators will be less effective, and stressed prey may be less able to avoid capture – and ultimately this stress will decrease survival, causing population size to suffer. For adults, stress caused by ocean acidification may not affect everyday activities, but it will ultimately reduce organisms' growth and reproduction rates. Decreased reproduction can also alter the entire population's size. Impacts at any life stage can reduce the potential for a population to grow or to recover from losses due to disturbance or stress. — Jim Barry, Senior Scientist. Monterey Bay Aquarium Research Institute, USA Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK

CO2 is a normal product of respiration. Animals breathe it in and out all the time. How can it possibly be toxic?
Just as in seawater, respiratory CO2 reduces the pH within cells. Organisms have evolved mechanisms to buffer, transport, and remove CO2 from their cells at the rate at which it is produced. Ocean acidification reduces the CO2 difference between the inside and outside an animal's body, thereby hindering CO2 removal and causing “respiratory acidosis.” (This term is analogous to “ocean acidification” because normal bodily fluids are slightly basic.) Respiratory acidosis may lead to, among other things, reduced metabolism and reduced organism activity. Additionally, many cellular functions are pH sensitive and may respond negatively to respiratory acidosis. For example, respiratory proteins (e.g. hemoglobin) in the blood bind oxygen at high pH and release it at low pH, allowing oxygen uptake at the gills and release at the cells, where metabolically produced CO2 has decreased local pH. Many organisms can compensate for respiratory acidosis by shifting the balance of ions in the body. However, it is unknown whether they can maintain such an ionic imbalance in the long term. — Brad Seibel, Assistant Professor of Biological Sciences, University of Rhode Island, USA

OA in geologic history

Why would coral become extinct because of ocean acidification, when coral species have already survived other ocean chemistry changes over geological history?
The danger from ocean acidification is related to the current rate of change, the concentration of atmospheric CO2 expected, and the magnitude of change of atmospheric CO2 forecast if we keep emitting CO2 at the same rate. The present rise in atmospheric CO2 is

2 ppm per year, and atmospheric CO2 has increased more than 100 ppm since the beginning of the Industrial Revolution. In the transition between the end of the last ice age to the current warm period, CO2 concentrations increased 80 ppm occurred over more than 10,000 years. Today’s rates of CO2 increase in the atmosphere are therefore approximately 100 times greater than most changes sustained over geologic time. Other than at times of the great mass extinctions, there is no evidence in the geologic record for sustained rates of change in atmospheric CO2 that have been as great or greater than today’s. Even during extreme ocean chemistry changes in geological history—- for example, during the Paleocene/Eocene thermal maximum 55 million years ago (Ma) when carbonate minerals dissolved in most of the deep and intermediate ocean—-these changes probably happened over several thousands of years. Corals have indeed survived multiple extinction events in Earth history, but each time their “rebound” took millions of years, and their ability to form reefs took even longer. The earliest corals arose during the Ordovician more than 400 million years ago. Known as Tabulate and Rugose corals, these were very different from the corals living today (modern corals belong to the Scleractinia and likely evolved independently from these earlier forms), and the Ordovician reef systems were dominated by sponges rather than corals. These groups went extinct during the Permo-Triassic extinction event 251 million years ago, and different coral lines eventually evolved and flourished again, along with reef-building bivalves that built tremendous reefs during through the Cretaceous period, most of which went extinct (along with the dinosaurs) in the Cretaceous extinction event 65 Ma ago. While coral reefs disappeared at this time, about half of all coral species did survive, but it took millions of years before reefs recovered to become widespread once again. In general, ocean life recovers from extinction episodes by adaptation and evolution of new species, but this takes roughly 10 million years to achieve pre-extinction levels of biodiversity. — Jim Barry, Senior Scientist, Monterey Bay Aquarium Research Institute, USA Daniela Schmidt, Senior Research Fellow, University of Bristol, UK Ken Caldeira, Senior Scientist, Carnegie Institution for Science, USA

How is today's change in ocean chemistry different from those of previous geological periods?
Present conditions differ from the past largely because the rate of change of atmospheric CO2 does not match the rate of mitigating geological processes. If CO2 is added slowly over hundreds of thousands of years, as it was during the Ordovician by volcanic and plate tectonic activity, the CO2 that enters the ocean has time to mix throughout the ocean from top to bottom. As a result, even though the amount of CO2 that is taken up by the ocean is large, it is spread out over a very great volume of water and the resulting decrease in pH is small. At the same time, as the CO2 level in deep oceans increases over millennia, carbonate sediments lying on the seafloor begin to dissolve and release carbonate ions that neutralize some of the acidity, further minimizing the decrease in pH. Past oceans also contained higher calcium and magnesium ion concentrations, which helped stabilize calcium carbonate minerals in marine animals’ skeletons.

Today, the CO2 in the atmosphere is increasing much faster than the ocean mixes. During CO2 releases like this over “short” (<10,000 year) timescales, the ability of sediments to regulate ocean chemistry is overwhelmed and both pH and saturation state decline. Even though the amount of CO2 that has entered the ocean in the last 200 years is smaller than that added during the Ordovician, the CO2 has built up to a much higher concentration in the surface ocean. As a result, upper ocean pH has decreased more rapidly and by a greater amount than in the geological past. Both the rate of change of pH and the magnitude of the change present problems for organisms that evolved in an ocean that experienced smaller, slower pH changes in the past. — Chris Langdon, Associate Professor, University of Miami, USA Andy Ridgwell, Royal Society University Research Fellow, Bristol University, UK Richard Zeebe, Associate Professor, University of Hawaii at Manoa, USA Daniela Schmidt, Senior Research Fellow, University of Bristol, UK

Scientific methods

Experiments on organisms are often unrealistic, because scientists sometimes add mineral acids and not CO2 to lower the pH to predicted levels.
When seawater is manipulated through addition of mineral acid, and this is accompanied by the addition of equal (equimolar) amounts of sodium bicarbonate, this approach perfectly simulates the changes in seawater carbonate chemistry induced by CO 2 uptake. Even when ocean acidification is simulated by manipulating seawater with mineral acids without adding bicarbonate (or carbonate), it is almost indistinguishable in terms of pH, pCO 2 , carbonate ion concentration, and saturation state from seawater manipulated through CO 2 aeration. The different treatments do result in slightly higher bicarbonate concentrations in CO 2 -aerated manipulations. However, bicarbonate increases in both approaches. In fact, no systematic difference is found in the responses of calcifying organisms exposed to seawater acidified by mineral acid or through CO 2 aeration. — Ulf Riebesell, Professor of Biological Oceanography, Leibniz Institute of Marine Sciences IFM-GEOMAR, Germany

Even scientists admit there are uncertainties about climate change. How certain is ocean acidification?
There is no argument that seawater chemistry is changing due to rising atmospheric CO2, and that human combustion of fossil fuels and deforestation are the root cause. There is less certainty about the possible biological impacts of ocean acidification, but this primarily reflects the fact that different groups of marine organisms express a wide range of sensitivity to changing seawater chemistry. There is broad agreement among the scientific community that ocean acidification is occurring and that it likely will have significant effects, some positive and some negative, on a large number of marine organisms. — Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA

The "evidence" about ocean acidification is conflicting, so even the scientists cannot agree.
There is no disagreement in the chemical data, which show that ocean acidification is happening. However, biological data show varied responses among organisms to OA. It can sometimes seem odd that experiments conducted on the same species can produce apparently contrasting results. However, it is clear that the response of marine organisms to elevated levels of CO2 is influenced not only by the organism's identity but also by the environmental conditions it has experienced during its life. Consequently, members of the same species collected from different areas, populations, or strains can exhibit different responses. This evidence should not be considered as conflicting, but rather an insight into the natural variability that exists among populations. Only by measuring and understanding this variability will we be better able to predict which species, communities, and ecosystems are at greatest risk from ocean acidification. — Steve Widdicombe, Benthic Ecologist, Plymouth Marine Laboratory, UK

Could the observed impacts of ocean acidification result from experiments that have simply placed them straight into water with CO2 levels that will take decades or centuries to reach, which amount to little more than a shock treatment?
In ocean acidification response experiments, animals are usually not placed immediately in CO2-enriched waters, but instead they are kept in water that is then equilibrated with carefully controlled gas mixtures. Although it is impossible to perform experiments that simulate the rate of anthropogenic CO2 accumulation in the atmosphere and the oceans, the CO2 levels used are far below those that have been shown to cause shock. Nonetheless, these CO2 levels may disturb physiological processes (acid-base regulation, development of larvae, growth) in ways that appear relatively mild on short time scales. Therefore, long-term exposures are usually needed to find out whether these levels are detrimental and cause fatalities. On long time scales, even small decreases in individual animals' health may harm a species, for example, in cases where species compete with others in ecosystems or when they are exposed to another stressor like extreme temperature. — Hans-Otto Pörtner, Professor, Alfred Wegener Institute, Germany

Geoengineering and mitigation

If we increase aquaculture and grow more shellfish, won't the shells help lock up carbon dioxide (like trees)?
The calcification process does take up carbon, but it causes shifts in the carbon system in seawater that result in a lower pH and an increase in CO2 rather than its removal. Many organisms convert bicarbonate to the carbonate they use to build their shells, and this produces hydrogen ions, thus increasing acidification. Most coral reefs, for example, on the time scales we are interested in, are small sources of CO2 to the atmosphere rather than sinks. From an ecosystems point of view, even well-intended aquaculture could cause unintentional harm by altering coastal landscapes, increasing pollution and disease, or releasing genetically altered or foreign species into the environment. Any activity aimed at reducing ocean acidification should be considered in a wider context to avoid replacing one environmental impact with another. — Anne Cohen, Research Specialist, Woods Hole Oceanographic Institution, USA Steve Widdicombe, Benthic Ecologist, Plymouth Marine Laboratory, UK

Can geoengineering solutions for climate change also help OA?
Most proposed geoengineering approaches to limit climate change attempt to provide symptomatic relief from climate change without addressing the root cause of the problem — excess carbon dioxide in the environment. Most geoengineering proposals address the climate consequences of our carbon dioxide emissions but do not address the chemical consequences of these emissions. For example, strategies that seek to cool the Earth by reflecting additional sunlight to space would have little direct effect on ocean chemistry and therefore would not significantly diminish ocean acidification.

Some proposals have sought to diminish changes in ocean chemistry by adding compounds to the ocean that would chemically neutralize acids. However, reversing ocean acidification this way would require adding an amount of material much larger than the amount of carbon dioxide we are emitting to the atmosphere. Therefore, these proposed solutions would require a new mining and chemical processing infrastructure as large as our current energy system. It seems reasonable to suggest that this level of effort and spending would be better applied to transforming our energy system away from dependence on a finite pool of fossil fuel resources to use of renewable, infinite resources— which would also prevent carbon dioxide from entering the environment in the first place instead of requiring us to consider neutralizing its effects after it is already spreading through the atmosphere and oceans. — Ken Caldeira, Senior Scientist, Carnegie Institution for Science, USA

Will capping atmospheric CO2 at 350 or 400 ppm stop OA?
Atmospheric CO2 is already at 390 ppm and is increasing at about 2 ppm per year. Without dramatic reductions in CO2 emissions, atmospheric CO2 will continue to rise, and most emission forecasts for the near future indicate a likely increase (rather than decrease) in atmospheric CO2 growth rate. The first step in addressing ocean acidification, therefore, is to stabilize and eventually reduce CO2 emissions. Atmospheric CO2 almost certainly will peak well above 400 ppm, because we will not stop increasing emissions in the next 5 years. The impacts on marine life at the peak CO2 level may be substantial. In the long run, it may be possible to reduce atmospheric CO2 through natural and artificial uptake mechanisms. The chemistry of seawater is reversible, and returning to 350-400 ppm would return pH and carbonate saturation levels to approximately their current conditions. However, some research has suggested that even current-day conditions may be deleterious for some organisms, and it is even less clear if future biological impacts due to peak CO2 will be reversible. Even if we stabilized CO2 emissions, atmospheric fossil fuel CO2 will continue to penetrate into the deep ocean for the next several centuries, which may impact deep water organisms such as cold-water corals. — Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA

Policy development and decisionmaking

Isn't it better that we sacrifice the oceans and let them keep on taking up CO2 and buffering climate?
Ocean acidification and climate change are two sides of the same coin. Both are direct consequences of anthropogenic CO 2 emissions and cannot be separated from each other. The present uptake of about one quarter of anthropogenic CO 2 emissions by the ocean indeed serves as a buffer against rising atmospheric CO 2 , and so this “service” could be considered to diminish, but not prevent, climate change. In the long term, on time scales of tens of thousands of years, the majority of anthropogenic CO 2 emissions (80-90%) will end up in the ocean. This, however, will not protect the climate system from global warming during the intervening period. It is also important to point out that the impacts of CO 2 uptake by the oceans will have profound effects on the functioning of Earth’s ecosystems. The oceans provide vital roles in biogeochemical cycles---not only in the regulation of CO 2 , but in the production of oxygen, the cycling of nitrogen and other important nutrients, as well as the production of gases that affect such things as cloud formation. Many species use both land and ocean habitats, and many humans rely on healthy oceans for their livelihoods. The oceans are an integral, interconnected part of the Earth system, and cannot be realistically considered as a separate entity. — Ulf Riebesell, Professor, Leibniz Institute of Marine Sciences IFM-GEOMAR, Germany Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA

Is it too late to do anything?
It is within our technical and economic means to modify our energy and transportation systems and land-use practices to largely eliminate carbon dioxide emissions from our economies by mid-century. It is thought that the cost of doing this — perhaps 2% of the worldwide economic production — would be small, yet at present it has proven difficult for societies to decide to undertake this conversion. — Ken Caldeira, Senior Scientist, Carnegie Institution for Science, USA

Why is it important to conduct research on OA? And what can scientists do?
Compared to our terrestrial environments, the oceans and their ecosystems are poorly understood. With recent technological advances, our knowledge is rapidly growing however, we still have much to learn. If policymakers are to make informed decisions regarding climate change and ocean acidification, scientists need to give them the best information possible. That requires research. Everyone must recognize that obtaining and distributing that knowledge takes a great deal of effort, and maintaining clear, open communication among researchers, leaders, and citizens is critical.

Scientists have answered the question “Is ocean acidification real?” --- yes. We are now confronted with the questions, “How bad will it be?” and, “What can be done?” Most scientists agree that reducing greenhouse gas emissions is the best answer to the latter. The remaining question is the most difficult yet most important question to answer, because we are keenly aware that CO2 levels will continue to rise in the foreseeable future. Many scientists are now focused on what CO2 concentration is considered “dangerous” to the planet and to society. Addressing “What can be done?” has shifted from what can be done about the cause of the problem, CO2, to what can be done about its consequences. Essentially, we seek to answer the question, “What will future marine ecosystems look like, and what ecosystem services will they provide to the planet and humankind?” This is a huge challenge. As evidenced by many of these questions, ocean acidification is a simple problem with complex consequences. — Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA Carol Turley, Senior Scientist, Plymouth Marine Laboratory, UK Robert Key, Research Oceanographer, Princeton University, USA

If ocean acidification is so potentially serious why isn?t it included in the United Nations Framework Convention on Climate Change (UNFCCC) Conference of the Parties (COP) climate mitigation negotiations?
Although scientists have known for decades that ocean acidification would occur as CO2 increased in the atmosphere, the consequences to marine life were not realized until about 10 years ago. At that time, biologists discovered that ocean acidification affected the ability of many marine organisms to form their shells or skeletons. Since then, many more effects of ocean acidification have been found to influence a wide array of organisms and marine processes. Because the scientific process relies on formal research protocols, peer-review, and publishing, it takes some time for a new finding to be verified and accepted by the scientific community. However, sufficient evidence about ocean acidification existed by 2007 that the IPCC Fourth Assessment Report on Climate Change (2007) stated in the Summary for Policy Makers, “The progressive acidification of the oceans due to increasing atmospheric carbon dioxide is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species.” Ocean acidification and its effects have now been documented to the point that they are widely accepted by the scientific community and it will be seriously addressed by the Fifth Assessment Report of the IPCC. In fact, ocean acidification was a major topic of discussion at side events such as Oceans Day at the December 2009 COP15 climate change negotiations in Copenhagen even though specific considerations about oceans had little or no mention in the text of the proposed agreement. — Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA Carol Turley, Senior Scientist, Plymouth Marine Laboratory, UK

Contributors and reviewers for this document

Jim Barry, Senior Scientist. Monterey Bay Aquarium Research Institute, USA
Jelle Bijma , Biogeochemist, Alfred Wegener Institute for Polar and Marine Research, Germany
Ken Caldeira, Senior Scientist, Carnegie Institution for Science, USA
Anne Cohen, Research Specialist, Woods Hole Oceanographic Institution, USA
Sarah Cooley, Postdoctoral Investigator, Woods Hole Oceanographic Institution, USA
Scott Doney, Senior Scientist, Woods Hole Oceanographic Institution, USA
Richard A. Feely, Senior Scientist, NOAA Pacific Marine Environmental Laboratory, USA
Helen Findlay, Lord Kingsland Fellow, Plymouth Marine Laboratory, UK
Jean-Pierre Gattuso, Senior Scientist, Centre National de la Recherche Scientifique and Université Pierre et Marie Curie-Paris 6, France
Jason Hall-Spencer, Marine Biology Lecturer, University of Plymouth, UK
Michael Holcomb, Postdoctoral Research Associate, Centre Scientifique de Monaco, Monaco
David Hutchins, Professor of Marine Environmental Biology, University of Southern California, USA
Debora Iglesias-Rodriguez, Lecturer, National Oceanography Centre of the University of Southampton, UK
Robert Key, Research Oceanographer, Princeton University, USA
Joan Kleypas, Scientist III, National Center for Atmospheric Research, USA
Chris Langdon, Associate Professor, University of Miami, USA
Daniel McCorkle, Associate Scientist, Woods Hole Oceanographic Institution, USA
James Orr, Senior Scientist, Laboratory for the Sciences of Climate and Environment, France
Hans-Otto Pörtner, Professor, Alfred Wegener Institute, Germany
Ulf Riebesell, Professor, Leibniz Institute of Marine Sciences IFM-GEOMAR, Germany
Andy Ridgwell, Royal Society University Research Fellow, University of Bristol, UK
Christopher L. Sabine, Supervisory Oceanographer, NOAA Pacific Marine Environmental Laboratory, USA
Daniela Schmidt, Senior Research Fellow, University of Bristol, UK
Brad Seibel, Assistant Professor of Biological Sciences, University of Rhode Island, USA
Carol Turley, Senior Scientist, Plymouth Marine Laboratory, UK
Steve Widdicombe, Benthic Ecologist, Plymouth Marine Laboratory, UK
Richard Zeebe, Associate Professor, University of Hawaii at Manoa, USA

Impacts of ocean acidification on marine shelled molluscs

Over the next century, elevated quantities of atmospheric CO2 are expected to penetrate into the oceans, causing a reduction in pH (−0.3/−0.4 pH unit in the surface ocean) and in the concentration of carbonate ions (so-called ocean acidification). Of growing concern are the impacts that this will have on marine and estuarine organisms and ecosystems. Marine shelled molluscs, which colonized a large latitudinal gradient and can be found from intertidal to deep-sea habitats, are economically and ecologically important species providing essential ecosystem services including habitat structure for benthic organisms, water purification and a food source for other organisms. The effects of ocean acidification on the growth and shell production by juvenile and adult shelled molluscs are variable among species and even within the same species, precluding the drawing of a general picture. This is, however, not the case for pteropods, with all species tested so far, being negatively impacted by ocean acidification. The blood of shelled molluscs may exhibit lower pH with consequences for several physiological processes (e.g. respiration, excretion, etc.) and, in some cases, increased mortality in the long term. While fertilization may remain unaffected by elevated pCO2, embryonic and larval development will be highly sensitive with important reductions in size and decreased survival of larvae, increases in the number of abnormal larvae and an increase in the developmental time. There are big gaps in the current understanding of the biological consequences of an acidifying ocean on shelled molluscs. For instance, the natural variability of pH and the interactions of changes in the carbonate chemistry with changes in other environmental stressors such as increased temperature and changing salinity, the effects of species interactions, as well as the capacity of the organisms to acclimate and/or adapt to changing environmental conditions are poorly described.

This is a preview of subscription content, access via your institution.


The youth of today are most likely to suffer the negative effects of global climate change. In order for the citizens of tomorrow to alter the present trajectory of our changing environment, they must bring global economies to task by imploring our leaders to implement sustainable management practices and make informed policy choices regarding the health of our planet. It’s vital to begin to create a connection between students’ general awareness of this problem and the actual consequences facing our natural world. Research has shown that understanding what global climate change is doesn’t necessarily lead to understanding the consequences of it (Boyes & Stanisstreet, 1993). Our perceptions of these issues often lack an experiential component – if we can’t “see” the negative effects of climate change, it is more difficult to understand what the impacts are. As advances are made in understanding how students learn and the impact of societal change in the 21st century, it is necessary to reexamine science programs (Bybee, 2012). The activities presented here are a step in the transition to a 21st-century approach to curriculum and instruction that builds students’ awareness through authentic science experience. These activities can be utilized to support teachers with transitioning to the type of teaching and learning outlined in the Next Generation Science Standards (NGSS Lead States, 2013). Additionally, teachers can use these investigations along with supplemental readings to integrate Common Core State Standards for Science and Technical Literacy.

Ocean Acidification: What Is It & Who Does It Affect?

Currently, the marine environment is undergoing a state of rapid change because of human activity (Meehl et al., 2007) as drastic increases in atmospheric concentrations of CO2 are absorbed by the world’s oceans (Sabine et al., 2004). Between 1751 and 1994, surface ocean pH is estimated to have decreased from 8.25 to 8.14 ( Jacobson, 2005), representing a

30% increase in hydrogen ion concentrations. A reduction in oceanic pH heavily influences marine organisms that produce shells and skeletons through the process of calcification, which involves the precipitation of dissolved ions into CaCO3, referred to as calcium carbonate (Fabry et al., 2008). Through the process of biomineralization, calcium carbonate is deposited within marine calcifying organisms and provides a physical structure for soft tissue to adhere to, in much the same way that bones provide structure for vertebrate organisms. When CO2 is absorbed by the ocean, resulting in decreased pH, the saturation state of CaCO3 is reduced, driving the dissolution of calcium carbonate within marine calcifiers, weakening the “bones” of these animals (Figure 1 for a review, see Fabry et al., 2008). Already, experimental work has found that ocean acidification negatively affects many species of marine calcifiers in all ecoregions, including coccolithophores, corals, foraminifera, echinoderms, crustaceans, and molluscs (National Research Council, 2011). Furthermore, the World Health Organization has stated that global climate change has already led to “5 million cases of illness and more than 150,000 deaths every year” (Patz et al., 2005), underscoring the need to broaden public awareness regarding the consequences of global climate change.

Discussions about global climate change in the classroom setting are often limited to effects on plants and animals. However, global climate change can also bring about conversations regarding how human-driven changes to our environment are affecting indigenous communities globally and locally. The threat that global climate change poses to many indigenous communities worldwide is particularly perilous, because traditional economies, cultures, and livelihoods are intrinsically and holistically linked to healthy marine ecosystems (Green et al., 2009). In the United States, Native communities have already begun to create and teach climate change–related curricula using traditional ecological knowledge as a framework for highlighting the importance of climate change education (Hugo et al., 2013). Therefore, climate change–related education influences not only the perceptions of the general public but also the health of indigenous people and preservation of indigenous ways of life (Green et al., 2009).

The Inky Abyss

In 1758, Carl Linnaeus, the Swedish taxonomist, was the first naturalist to classify and name the cold-water coral Lophelia pertusa after fishermen had inadvertently captured it with nets and hooks in Scandinavian fjords.

By the early nineteenth century, British fishermen were operating steam trawlers with power winches to pull up the catch. With this technology, fishermen could fish in somewhat deeper water and also accidentally grab corals. The scientist Louis Agassiz brought up cold-water coral specimens by dredging off the U.S. Southeast in 1880.

Still, it was relatively rare for hooks and trawls to grab cold-water corals. Fishing and research technologies of the time could reach only moderate depths, and scientists believed that cold-water corals were geographically limited and few in number. It seemed very unlikely—even impossible—that vast coral reefs around the world could survive in conditions of unending darkness, intense pressure, and refrigerator chill.

But in the mid-1990s improved technologies—acoustic surveys, Remotely Operated Vehicles (ROVs), and manned submersible craft—allowed scientists to study stretches of the world’s deeper regions. Still, powerful currents, rough terrain, intense pressure, extreme darkness, and high costs of ship time and deep-sea technologies have continued to make deep-sea exploration difficult.

Scientists have mapped deep-sea corals in the Gulf of Mexico and Mediterranean, along continental slopes on both sides of the Pacific and Atlantic, and on hundreds of undersea mountains called seamounts in the open ocean. Seamounts often peak 300 meters beneath the ocean surface.

Underwater Investigations. A view from inside the Johnson-Sea-Link submersible near the top of a deep-sea coral mound off Cape Fear, North Carolina, in October 2005. Scientists examined thickets of the cold-water coral Lophelia pertusa at a depth of about 370 meters. Photo by Art Howard.

Today, most of the nation’s cold-water corals remain hidden in an unexplored world. Less than 10 percent of the sea bottom of the entire U.S. Exclusive Economic Zone—three miles to 200 miles from shore—has been mapped with low-resolution tools, which provide a general typography of the bottom, says Kimberly Puglise, science director of NOAA’s Office of Ocean Exploration and Research.

Less than five percent of potential deep-sea coral habitat from North Carolina to eastern Florida has been mapped with high-resolution technologies, which provide detailed images of seafloor resources such as corals and rocks. To study and manage cold-water corals, NOAA would need further high-resolution images of the deep-sea floor.

“We just don’t know enough about where most deep-sea corals are” in U.S. marine waters, says Puglise. “That makes it difficult to manage their use.” NOAA is working on a strategic plan for deep-sea corals and sponges that would address both further research and management. It will be available for public comment in 2009. The South Atlantic Fishery Management Council has already convened experts to write a research plan for deep-sea corals of the region.

Federal budgets for deep-sea explorations nationwide have essentially been reduced since the late-1990s, limiting scientists’ opportunities to explore more of the nation’s deep-sea bottom and to provide the “best available science” required for fishery management as mandated by federal law, says Steneck.

U.S. funding of deep-sea explorations has been “at pittance scale compared to those of shallow-water corals,” says Rader. “We are badly in need of documentation of what’s down there. Still, it’s amazing what scientists have been able to do on a shoestring in the Southeast.”

Explorations of Southeast’s deep-sea corals have generated both public excitement and concern over the fate of these unusual resources.

Delicate Habitat. There are likely thousands of deep-sea coral mounds from North Carolina to east Florida. Across the surface of the mounds, the coral Lophelia pertusa forms dense bushes and thickets on the seafloor but also elegant, solitary trees like this one (top). Cnidarian coral colonies (bottom) also formed this tree-like structure. Photos by S.W. Ross, et al., Leslie R. Sautter.

In May 2008, South Carolina Gov. Mark Sanford wrote a letter to the White House asking the federal government to designate the deep-sea coral region off the southern Atlantic seaboard as a marine national monument. Monument status would provide comprehensive government protection for the corals of the region. More than 120 scientists also petitioned the president to consider protecting it as a national monument.

In 2006, conservationists praised President George W. Bush for naming a nearly 140,000-square-mile marine reserve in the Northwestern Hawaiian Islands as a national monument.

But when gasoline prices rose during the summer of 2008, there were calls for additional offshore oil and gas exploration in U.S. waters. A federal moratorium had prohibited drilling for fossil fuels along most of the U.S. coastline, including the southern Atlantic seaboard, until 2012. But the moratorium was recently withdrawn.

The deep-sea coral banks of the U.S. southern Atlantic coast did not make the White House final list of areas to be nominated for monument status. Instead, three sites in the Pacific Ocean have been designated.

Rader says that the South Atlantic Fishery Management Council’s proposed HAPCs could provide nearly as much protection as a monument designation while also allowing some fishing to continue. “Once the Council acts,” he says, “the biggest challenge will be finding the resources needed for enforcement, outreach and education, and research and monitoring of these world treasures.”


Seawater chemistry

Mussel larvae cultured in jars equilibrated with the three different atmospheric CO2 concentrations experienced significantly different seawater chemistries during their pelagic phase (ANOVA, F2,15=34015, P<0.0001 Fig. 2A). Mean pH across all 380 ppm control jars was 8.06±0.001, whereas the 540 and 970 ppm jars had mean pH values of 7.96±0.001 and 7.75±0.001, respectively. TA differed only slightly among CO2 levels, as would be expected, with the 380 and 970 ppm treatments exhibiting values of 2223±0.15 and 2228±0.29 μmol kg –1 seawater, respectively (ANOVA, F2,15=102.34, P<0.0001 Fig. 2B).

Checks on internal consistency of the carbonate system demonstrated that pHNBS and seawater PCO2 calculated from TA and DIC were within 1.2 and 8.6% of measured values (means within 1.0 and 6.6% Table 1), respectively. Calculated pHNBS values were consistently 0.08 units higher than those measured, corresponding to a ∼17% discrepancy in [H + ] due to the logarithmic nature of the pH scale. Such offsets are not unusual for potentiometric pH measurements given that pHNBS values can displaced from absolute levels by magnitudes approaching 0.1 units, mostly from large inter-electrode differences in liquid junction potential (Dickson, 1984). The offsets do not influence the validity of relative comparisons among pH data acquired, as in the present study, by means of a single electrode.

Mean (±s.d.) carbonate system parameters during the larval culturing

XyUfgGw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA" />

Representative strength test of a larval mussel shell, showing how force increased linearly with shell compression. Compressive forces were intended as a mimic to crushing attacks by predators such as crabs. However, because failure in biomaterials often varies with the speed and geometry of force application, and with hydration state, results are unlikely to duplicate natural predatory interactions in all respects. The slope of the trace does not provide a measure of shell stiffness because of compliance in the testing apparatus.

Representative strength test of a larval mussel shell, showing how force increased linearly with shell compression. Compressive forces were intended as a mimic to crushing attacks by predators such as crabs. However, because failure in biomaterials often varies with the speed and geometry of force application, and with hydration state, results are unlikely to duplicate natural predatory interactions in all respects. The slope of the trace does not provide a measure of shell stiffness because of compliance in the testing apparatus.

Effects on shell strength

Cultured larvae placed in the materials testing apparatus sustained linearly increasing compressive forces until a point of catastrophic shell failure (Fig. 3). As is often seen in mechanical tests, the force required to induce breakage varied appreciably among individuals. Coefficients of variation for breaking force ranged from 18 to 43% in a given culture jar. Mean strength of the larval shells also increased over the course of development, with breaking forces from day 8 exceeding those from day 5 by over a factor of two (Fig. 4). Note that one 380 ppm control culture developed a filamentous algal bloom and the mean shell strength of this jar failed outlier tests (Dixon's r10=5.58, N=6, P<0.05 Grubb's G=1.83, N=6, P<0.05). This jar was therefore discarded from subsequent analyses.

The degree of seawater acidification strongly impacted shell strength on both day 5 and day 8 after fertilization (ANOVA, day 5, F2,15=4.47, P=0.03 day 8, F2,14=6.48, P=0.01 Fig. 4). At day 5, larvae reared in the 540 ppm CO2 treatment had shells that were 13% weaker than those of control individuals, and individuals reared in the 970 ppm CO2 treatment had shells that were 20% weaker. At day 8, the corresponding reductions in shell strength were 12 and 15%.

Impacts on shell area

Ocean acidification had a much smaller effect on larval shell growth (Fig. 5) compared with its impact on strength. At day 5, larvae reared in the 970 ppm CO2 treatment had shells that were 7% smaller in area than those of control individuals. At day 8, the corresponding reductions in shell area were 5%. The decreases in growth were not the principal cause of the reductions in shell strength, as there was no relationship between shell area and strength on a given day. In addition to the main treatment effect, the pattern of variation in shell area among jars also implied a significant effect of the nested factor airstream [CO2] in the mixed-model ANOVA (day 5 CO2, F2,12=37.94, P<0.0001 day 5 airstream [CO2], F3,12=3.94, P=0.04 day 8 CO2, F2,11=16.59, P<0.0005 day 8 airstream [CO2], F3,11=13.55, P<0.0005). However, further examination suggested that the nested effect actually resulted from jars of a given airstream being located in a specific table. In particular, shell areas of larvae reared in the second of the two tables were uniformly smaller, regardless of CO2 level (Fig. 5), and no airstream-associated differences in water chemistry were evident. Consistent with the interpretation of a table effect, a two-way ANOVA with CO2 and table as fixed factors indicated a significant effect of both factors on shell area, with no interaction between them, for both day-5 and day-8 larvae (day 5 CO2, F2,14=40.26, P<0.0001 day 5 table, F1,14=11.28, P<0.005 day 8 CO2, F2,13=19.28, P=0.001 day 8 table, F1,13=46.17, P<0.0001 Fig. 5). These results suggest a subtle impact of CO2 on growth rivaled by minor experimental effects associated with use of a particular seawater table. Although possible causes of a table effect are uncertain, mussel growth varies with seawater temperature (Widdows, 1991), and the two tables differed in mean temperature by ∼0.2°C. Such apparent sensitivity to experimental conditions indicates a place for substantial care in the design of OA culturing studies.

Shell breaking force as a function of CO2 treatment at (A) day 5 and (B) day 8 post-fertilization in mussel larvae. Shared letters above bars in each panel indicate shell breaking forces that did not differ (Tukey's HSD, P<0.05). Error bars denote +s.e.m.

Shell breaking force as a function of CO2 treatment at (A) day 5 and (B) day 8 post-fertilization in mussel larvae. Shared letters above bars in each panel indicate shell breaking forces that did not differ (Tukey's HSD, P<0.05). Error bars denote +s.e.m.

Shell area as a function of CO2 treatment at (A) day 5 and (B) day 8 post-fertilization for mussel larvae cultured in each of two seawater tables (black and white bars for each CO2 level). Shared letters above paired bars in each panel indicate mean shell areas that did not differ among CO2 treatments (Tukey's HSD, P<0.05). Note that shell areas differed subtly but significantly between larvae raised in the two seawater tables (P<0.05). Error bars denote +s.e.m.

Shell area as a function of CO2 treatment at (A) day 5 and (B) day 8 post-fertilization for mussel larvae cultured in each of two seawater tables (black and white bars for each CO2 level). Shared letters above paired bars in each panel indicate mean shell areas that did not differ among CO2 treatments (Tukey's HSD, P<0.05). Note that shell areas differed subtly but significantly between larvae raised in the two seawater tables (P<0.05). Error bars denote +s.e.m.

Effects of acidified seawater on additional day-8 larval parameters potentially influencing early-life survivorship of mussels. (A) Mean larval shell thickness. (B) Mean dry tissue mass. (C) Mean ratio of surface area to dry tissue mass. Shared letters above bars in each panel indicate parameters that did not differ among CO2 treatments (Tukey's HSD, P<0.05). Error bars denote +s.e.m.

Effects of acidified seawater on additional day-8 larval parameters potentially influencing early-life survivorship of mussels. (A) Mean larval shell thickness. (B) Mean dry tissue mass. (C) Mean ratio of surface area to dry tissue mass. Shared letters above bars in each panel indicate parameters that did not differ among CO2 treatments (Tukey's HSD, P<0.05). Error bars denote +s.e.m.

Effects on shell thickness and tissue mass

Other factors tied to larval shell strength and growth were also impacted by acidification. Shell thickness was significantly altered under OA (ANOVA, F2,15=4.29, P=0.03 Fig. 6A), leading to larval shells at day 8 that were approximately 15% thinner in elevated CO2 treatments compared with the control. The condition index (ratio of ash-free dry tissue mass to total dry mass) of larvae varied significantly across treatments (ANOVA, F2,13=8.49, P=0.004), with larvae from the 970 ppm treatment exhibiting values that were 17% lower than those of larvae from 540 ppm and control cultures. Note, however, that because shell mass enters into the calculation of the condition index and was reduced in the elevated-CO2 treatments, dry tissue mass may provide a more direct measure of the energetic status of larvae. Dry tissue mass varied across CO2 treatments (ANOVA, F2,13=9.93, P=0.002 Fig. 6B), with values for larvae from the 540 and 970 ppm treatments reduced by 14 and 33%, respectively, relative to that of the control. The relationship between shell area and dry tissue mass differed substantially across treatments (ANOVA, F2,13=12.88, P=0.0008 Fig. 6C), with larvae cultured in 970 ppm CO2 seawater exhibiting an area/mass ratio that was 40% larger than that of control larvae.

Evolutionary response possibilities

Organismal responses to changing ocean temperature and pH are likely to involve plasticity where one genotype can produce a range of phenotypes or adaptation by local or widespread selection of existing genetic diversity (Sunday et al., 2014). An evolutionary ‘time machine’ may exist in freshwater ecosystems, whereby diapause embryos of Daphnia magna trapped in sediments produced during pre-industrial atmospheric PCO2 can be resurrected for study (Orsini et al., 2013). No such dormant propagules of marine animals are available to assess evolutionary changes in marine ecosystems, although algae cysts in sediments may present that opportunity (Ellegaard et al., 2013 Härnström et al., 2011). However, the ‘crystal ball’ of the comparative physiology approach can inform us as to how organisms are likely to respond given present phenotypes (Somero, 2010, 2011).

Three main approaches have been used to assess adaptive potential to ocean acidification (Sunday et al., 2014): (1) Short-term acclimation studies (within one generation) to assess the capacity for physiological plasticity (2) population or species level comparative studies to assess extant genetic and physiological diversity in organisms distributed across natural gradients in pH and (3) experimental evolution, whereby selection of extant genetic diversity or novel mutations can lead to shifts in performance over multiple generations. Here, we describe advances in each type of approach related to understanding whether biochemical systems are adapted to environmental gradients in pH.


Plastic responses to ocean acidification involve those made within the lifetime of an individual (Somero, 2011), or potentially that persist for multiple generations through epigenetic or maternal effects (Burggren, 2014). Studying organismal responses to anthropogenic changes in the ocean pH yields insight into physiological systems most sensitive to habitat pH and with the potential for adaptation to ocean acidification. Organisms exposed to OA exhibit a wide array of responses that vary across response variables, life history stages, geographic locations and taxa. Meta-analyses have revealed across taxa that survival and calcification are most negatively affected (Kroeker et al., 2013). Taxon-specific analysis reveals the most heavily calcifying groups (calcified algae, corals, mollusks and the larval stages of echinoderms) are most negatively affected by decreases in pH (Kroeker et al., 2013). It is likely that there are environmental pH threshold limits to plastic responses, beyond which acclimation does not occur and fitness is impaired (Dorey et al., 2013).

Reductions in pH disrupt fish olfactory senses, which reduces the ability of individuals to detect predators, prey and parental cues (Dixson et al., 2010 Munday et al., 2009 Nilsson et al., 2012). These changes are mediated through the direct effects of pH regulation on GABA neurotransmitter pathways. Shifts in acid–base regulation alter the gradients of anions (Cl − and HCO3 − ) in neuronal membranes, resulting in a reversed current flow through GABA receptors (Hamilton et al., 2014 Nilsson et al., 2012). The impacts of reduced pH on fish olfaction can persist across generations (Welch et al., 2014), suggesting limited phenotypic plasticity in physiological systems involving olfaction. Olfactory neuron architecture is probably similar across fish. Thus, future studies that compare the acid–base regulatory physiology and GABA receptor responses in fish showing different behavioral responses to pH could illuminate an important example of biochemical adaptation to ocean acidification. Such studies could compare across individuals with different behaviors in response to changes in environmental pH (Welch et al., 2014) or among fish adapted to different pH environments.

An important consideration is that pH is not changing in isolation. Increased climate warming due to elevated atmospheric PCO2 is concomitantly changing sea surface temperature and those factors must be considered in combination for ecologically and evolutionarily realistic conclusions to be drawn (Harvey et al., 2013). The effect of OA is generally interactive with the effect of a temperature (Harvey et al., 2013). Physiological energetics may be the basis for the interactive effects, because shifts in energy partitioning between growth and maintenance have been observed for each environmental driver independently. Interactive effects of acidification and temperature variation have been observed in the intertidal porcelain crab, Petrolisthes cinctipes (Paganini et al., 2014). Acclimation to increasing pH variability caused an elevation in thermal tolerance, but no concomitant increase in respiration rate (Paganini et al., 2014). In contrast, pH variability had strong interactive effects with the effect of temperature on metabolic rate and thermal tolerance, and thus, sensitivity to pH is context dependent (Paganini et al., 2014). For coastal organisms living in dynamic pH habitats (Duarte et al., 2013), incorporating responses to environmental variability in pH and temperature is an important aspect of understanding plastic and adaptive responses (Dupont and Portner, 2013).

Comparative studies

Populations living across natural ecological gradients in pH demonstrate that decreased calcification under low pH is a real-world phenomenon, not just one that is observed in laboratory experimentation. For example, in situ shell dissolution of the coastal snail Limacina helicina is accelerated at relatively more-acidified sites in the California Current Large Marine Ecosystem (CCLME) (Bednarŝek et al., 2014). Sea urchin species distributed across pH gradients at naturally occurring CO2 vents in the Mediterranean Sea have shown that local adaptation to high PCO2 environments leads to an adaptive response, resulting in a high buffering capacity of intracellular fluid (Calosi et al., 2013). In this case, this physiological differentiation is what leads to the species distribution patterns. Interspecific comparisons of species adapted to different pH habitats offer insight into how organisms differ in their tolerance for such changes. Organismal tolerance for pH stress is shown when comparing congeners from habitats with different selective pressures, for example, porcelain crabs distributed across the intertidal–subtidal vertical gradient (Stillman and Somero, 2000). Comparisons of porcelain crabs in the genus Petrolisthes show how less-thermally-tolerant species exhibit higher mortality with concurrent decreases in exoskeleton [Ca 2+ ] when exposed to pH stress than congeners from more stable (e.g. subtidal) temperature habitats (Page and Stillman, 2014). Variation in coccolithophore calcification has been seen across species and strains distributed across global-scale physico-chemical gradients, whereby coccolith mass is inversely correlated with PCO2 (Beaufort et al., 2011), highlighting the importance of comparing similar taxa distributed across large gradients in ocean chemistry conditions in order to understand adaptive potential to ocean acidification.

Intraspecific comparisons between individuals from different habitats allow inferences to be drawn about the plasticity of the physiological responses organisms may have to OA. Local adaptation and differential selection of specific genotypes under acidified conditions has been shown to govern allele frequency of top candidate genes for OA responses (Pespeni et al., 2013). Purple urchin larvae (Strongylocentrotus purpuratus) locally adapted to less-acidic sites show this increase in allele frequency when exposed to pH stress, indicating that the adaptive capacity may be a result of standing genetic variation across the spatial-temporal habitats (Pespeni et al., 2013). Intraspecific comparisons with differing thermal habitats show how thermal plasticity can shape responses to pH stress. Populations of intertidal Concholepas concholepas snails from warmer habitats increase their aerobic capacity, resulting in higher levels of molecular chaperones when exposed to pH stress (Lardies et al., 2014). These responses are indicative of how thermal plasticity across populations can govern the tolerance limits of acidification stress, possibly by inducing similar pathways.

Similarly, diversity of phenotypic plasticity within populations can have significant effects on how species respond to OA. Brood-specific responses have been shown to be beneficial in regards to OA (Carter et al., 2013 Ceballos-Osuna et al., 2013), which are especially vital since early life stages (e.g. embryonic, larval) can be the most vulnerable to environmental stress (Miller et al., 2013). In porcelain crab larvae and embryos, individuals from some broods show a metabolic reduction in response to lowered pH, whereas in other broods, the same life stages are largely unaffected (Carter et al., 2013 Ceballos-Osuna et al., 2013). Brood-specific variation in response to lowered pH could be due to genetic variability among parents, though maternal effects related to environmental exposure during or prior to oogenesis could also play a role differentiating between genetic and epigenetic or maternal effects remains an important challenge in determining the sources of plasticity.

Increased phenotypic and genetic variation for larval size of coastal invertebrates in future CO2 conditions has been shown to be key in understanding relative evolutionary potentials across a large number of species (Sunday et al., 2011). Increases in larval size can produce faster evolutionary responses to pH stress despite having lower rates of population turnover (Sunday et al., 2011). On a population level, the degree to which phenotypic plasticity is an important aspect of tolerance to ocean acidification may be related to standing genetic diversity. For example, increased tolerance for acidification is shown in urchins (Foo et al., 2012 Kelly et al., 2013) because of standing genetic diversity.

The capacity for adaptive responses through selection of genetic variation that leads to acidification-tolerant phenotypes has been demonstrated in many taxa, principally through studies where breeding designs (e.g. North Carolina breeding design) allow for partitioning of phenotypic diversity into genetic and environmental components (Lynch and Walsh, 1998). This approach has been useful for identification of sea urchin genotypes that produce embryos that are more resistant to OA and warming, potentially as a result of maternal provisioning (Foo et al., 2012), and larvae with growth that is less impacted by OA (Kelly et al., 2013 Sunday et al., 2011). Variation in responses to OA and OW at urchin early life stages are also evidenced at the molecular level, through differential regulation of gene expression (Evans et al., 2013 Padilla-Gamiño et al., 2013 Todgham and Hofmann, 2009), which has also been shown in abalone (Zippay and Hofmann, 2010).

Mechanisms that are responsible for calcification are paramount to also understanding adaptive shifts to OA. Purple urchin larvae, Strongylocentrotus purpuratus, reared under high PCO2 were found to exhibit broad-scale decreases in gene expression in four major cellular processes: biomineralization, cellular stress response, metabolism and apoptosis underscoring that physiological processes beyond calcification and biomineralization are impacted greatly (Todgham and Hofmann, 2009).

Oysters are economically important organisms that have had a huge influence on the attention given to ocean acidification in the public sector because of the sensitivity to OA during their early life stages (Barton et al., 2012). When water in oyster hatcheries is acidified, largely because of variation in pH across the CCLME, early ‘ D ’ stage larvae suffer high mortality (Barton et al., 2012). Selective breeding of oysters has great potential to diminish OA impacts on growth and energetics by selection of genotypes that are most fit under future OA conditions (Applebaum et al., 2014 Parker et al., 2011). However, there are potentially trade-offs in oyster biology between being well adapted to OA and other life history characteristics. For example, bryozoan clonal isolates exhibited correlated life history traits and trade-offs of those traits with tolerance to OA and warming (Pistevos et al., 2011). Clearly, there is much that remains to be learned about correlated traits that may have fitness consequences (or advantages) in addition to tolerance to OA conditions. Demonstration of the potential for existing genetic diversity contributing to the resilience of these and other species to a changing ocean suggests that conserving locally adapted populations in low- or variable-pH environments should be emphasized.

Experimental evolution

Adaptive responses to OA in marine organisms have focused on phytoplankton, including diatoms and coccolithophores, because of their importance in the ocean's food webs and biogeochemical cycles and their short generation times (Tatters et al., 2013 Falkowski, 2012). Coccolithophores are generally thought to have reduced inorganic carbon content under OA, though laboratory studies indicate a remarkable diversity in the responses of individual genotypes to future conditions (see Benner et al., 2013 for review). Because coccolithophores can be cultured for hundreds of generations under controlled conditions, they have been used in studies of experimental evolution to assess their adaptive potential to OA (Reusch and Boyd, 2013). The coccolithophore Emiliania huxleyi is a particularly well studied species, with remarkable diversity in how it responds to OA across genotypes (Langer et al., 2009). Studies of specific strains held under different conditions for different lengths of time illustrate the potential for coccolithophores to make plastic and adaptive responses to OA (Benner et al., 2013 Langer et al., 2009 Lefebvre et al., 2012 Lohbeck et al., 2012, 2013 Schlüter et al., 2014). E. huxleyi typically exhibited a plastic response after 8 generations at high PCO2 and an adaptive response after 500 generations under the same high PCO2 conditions (Lohbeck et al., 2014). After 500 generations under OA, E. huxleyi can adaptively regulate genes responsible for cytosolic pH regulation (upregulation of proton pumps and bicarbonate transporters) and subsequently increase its growth and calcification (Lohbeck et al., 2012, 2013, 2014). Shifts in cell size have been observed following experimental evolution under elevated PCO2 in the freshwater green algae Chlamydomonas (Collins and Bell, 2004). The accumulation of mutations in genes involved with carbon-concentrating mechanisms is believed to be responsible for the adaptive shifts in Chlamydomonas (Collins and Bell, 2004).

Marine organisms live in a complex multi-driver environment and in the future, phytoplankton are likely to have to cope with OA concomitantly with warming, shifts in the nitrogen cycle, and potentially other environmental changes. Future PCO2 levels are expected to increase the NH4 + /NO3 ratio in surface waters via a doubling of N2 fixation rates by Trichodesmium (Barcelos e Ramos et al., 2007 Hutchins et al., 2009). Calcification and carbon fixation of E. huxleyi are more sensitive to nitrogen source (NH4 + versus NO3 − ) than elevated PCO2, (Lefebvre et al., 2012). Those two environmental drivers interactively alter the ratios of calcification and photosynthesis products of particulate inorganic and organic carbon (Lefebvre et al., 2012). Interestingly, warming seems to ameliorate the negative effects of OA under long-term culture (Benner et al., 2013). The multivariate responses to changes in temperature, PCO2 and nitrogen source remain to be examined.

Studies on non-calcifying phytoplankton have also provided insight into the possible effects of OA on community interactions and productivity. For example, community structure in a mixed dinoflagellate assembly did not shift under OA conditions in a manner that suggested adaptation or acclimation of individual community members (Tatters et al., 2013). Rather, increases in the fitness of specific strains were attributed to biotic interactions (Tatters et al., 2013).

… and sizes

Shell size can in part be related to the environment where it grows. Some (but not all!) shellfish from colder regions, closer to the poles, are very slow-growing, and so don’t grow very big. They can, however, live for a long time—the oldest known individual animal is actually a shellfish—the ocean quahog, Arctica islandica. This shellfish lives in the cold waters of the north Atlantic, and the growth banding of one shell sample showed that it lived to be 507 years old.

Arctica islandica Image source: Willow Herb / Flickr.

Warm, tropical locations are home to giant clams (Tridacna spp.)—which can reach more than 1.2 metres in size—and also to the tiny shells that help make up the sand on some beaches on coral atoll islands. The tiniest mollusc shell is that of a snail that lives on limestone cliffs in Malaysian Borneo—it has an average height of 0.7 millimetres.

Speaking of tiny, it would be remiss to overlook the huge range of microorganisms that live in the ocean that also build shells of calcium carbonate. These are the foraminifera (affectionately known as forams) and are present in ocean sediments, the ocean water column and other aquatic environments. All in all, there are over 50,000 foram species—10,000 living and another 40,000 documented within the fossil record. Of the living species, only around 40 species live within the water column and the rest live within the sediments of the sea (or lake, or river) bed. Although these animals are tiny, they also build extremely elaborate and beautifully structured shells, and they play a crucial role in cycling carbon through the planet’s oceans.

Microscopic image of forams. Image source: Chris Moody / Flickr.

Another important shell-maker in the ocean that is extremely important to the global carbon cycle are coccolithophores. These guys are actually single-celled plants. They are also microscopic, and are formed from several circular ‘plates’ layered over each other to form a spherical shape. Individual plates are around three one-thousandths of a millimetre in diameter. There is a ridiculous amount of coccolithophores in the ocean—there can be two generations produced in a single day and it’s estimated these tiny creatures create more than 1.4 billion kilograms of calcite per year.

Satellite imagery of coccolith bloom over Barents Sea, Arctic Ocean. Image source: Jeff Schmaltz / Nasa Earth Observatory.

And to complete the trifecta of tiny, we have ostracods. These are tiny crustaceans that also form a calcite shell. There are around 70,000 known species of ostracods, 13,000 of which are still living, the others found only in the fossil record. Ostracods are found in both marine and freshwater environments.

Soft imaging of ostracod shell. Image source: Hecht801 / Flickr.

Just as bigger shells can record environmental conditions in the structure and chemistry of their shells, so do foraminfera, coccolithophores and ostracods. Specimens preserved throughout the fossil record have provided a wealth of information that scientists have used to determine Earth’s climatic conditions in the far distant past. It was by studying the chemistry of forams that scientists were able to put together the Earth’s history of glacial cycles (Ice Ages)—tiny shells that told a huge story.

Other shells that are important in the fossil record are brachiopods. There are still some species of brachiopods that exist today, but not many—they have largely been out-competed through geological history by the mollusc species that are common today. They are extensive in the fossil record though, and their shells can also provide important climate information.

And although it’s these beautiful, intricate shells, replete with information, that are the legacy these animals leave behind, shellfish animals themselves have played an important role in human history and evolution. Shellfish have been an important part of the Hominin diet for more than a million years. Homo erectus in Trinil, Java, H. erectus in the Levant at Ubeidiya, Neanderthals in Gibraltar and early H. sapiens at Pinacle Point in South Africa all feasted on seafood buffets back in the day. It’s thought that the micronutrients (e.g. Omega 3s/DHAs) from these foods likely played an important role in brain evolution. So we can thank shellfish for not only providing a record of climatic history within their shells, but also for enabling us to be smart enough to figure it out!

All in all, the diversity in size, shape and colouring of the shells is nothing short of amazing, and quite a few mysteries remain regarding the mechanics of exactly how these animals create their strong, lightweight and durable homes.